Background
The author sets the stage for his study of the question that is the title of his Nature letter by stating that a fundamental assumption of the Intergovernmental Panel on Climate Change and many energy analysts is that "each unit of energy supplied by non-fossil-fuel sources takes the place of a unit of energy supplied by fossil-fuel sources." However, he goes on to say that "owing to the complexity of economic systems and human behavior, it is often the case that changes aimed at reducing one type of resource consumption, either through improvements in efficiency of use or by developing substitutes, do not lead to the intended outcome when net effects are considered," citing Sellen and Harper (2002), York (2006), Polimeni et al. (2008), Druckman et al. (2011) and Hubacek and Guan (2011).

What was done
Using an approach that controls for the principal driving forces of national per capita demand for fossil-fuel energy derived from coal, gas and oil, and which also tests for displacement, York accounted - in four different models - for the amount of energy per capita that is obtained from non-fossil-fuel sources (hydropower, nuclear, geothermal, solar, wind, tidal and wave energy, combustible renewables and waste) and that is measured in the same units of energy as that obtained from fossil-fuel sources.

What was learned
In regard to the question posed in the title of his paper - Do alternative energy sources displace fossil fuels? - York says that the answer is "yes, but only very modestly," noting that "across most nations of the world over the past fifty years, each unit of total national energy use from non-fossil-fuel sources displaced less than one-quarter of a unit of fossil-fuel energy use and, focusing specifically on electricity, each unit of electricity generated by non-fossil-fuel sources displaced less than one-tenth of a unit of fossil-fuel-generated electricity."

What it means
In the words of the University of Oregon (USA) researcher, "these results challenge conventional thinking in that they indicate that suppressing the use of fossil fuel will require changes other than simply technical ones such as expanding non-fossil-fuel energy production." Apparently, there is nothing that quite compares with good old coal, gas and oil in terms of the "bang for the buck" that they provide.